The invention relates to a process for increasing the proportion of components which are non-volatile under processing conditions, in basecoat compositions containing predominantly organic diluents or solvents, for the production of multicoat finishes, preferably metallic finishes.
In the area of automotive finishing in particular, but also in other areas, considerable interest centers on basecoat compositions for the production of multicoat finishes.
Multicoat finishes of the basecoat-clearcoat type have gained wide acceptance in automotive finishing, in particular for metallic finishes.
Basecoat-clearcoat finishes are produced in such a way that a pigmented basecoat is applied first, and after a short flash-off period without baking (the wet-on-wet process), a clearcoat is applied over the basecoat and subsequently the basecoat and clearcoat are baked together.
The paints for the production of these basecoats must be capable of being processed by the today""s customary economical wet-on-wet process, ie. they must be capable of being coated by a (transparent) topcoat after a predrying period as short as possible without baking, without manifesting interfering dissolution and strike-in phenomena.
In addition, still further problems must be solved in the development of paints for metallic finish basecoats. The metallic effect depends crucially on the orientation of the metallic pigment particles in the paint film. A metallic finish basecoat suitable for the wet-on-wet process must therefore produce paint films in which the metallic pigments are present, after application, in a favorable spatial orientation and in which this orientation is rapidly fixed in such a way that it cannot be disturbed in the course of the coating process.
A whole range of basecoat compositions containing predominantly organic diluents or solvents for the production of multicoat finishes, in particular metallic finishes, is known which is suitable for producing multicoat finishes with excellent properties.
However, these basecoat compositions belonging to the state of the art have the disadvantage of containing a relatively low proportion of components which are non-volatile under the processing conditions.
It is a declared aim of paint manufacturers to reduce the proportion of components which are volatile under processing conditions in basecoat compositions containing predominantly organic diluents or solvents, for the production of multicoat finishes, preferably metallic finishes.
There has been no shortage of experiments aiming at increasing the proportion of non-volatile components at the expense of the volatile components.
Thus attempts have been made, for example, to reduce the mean molecular weight of the binders contained in the basecoat compositions and/or to add microgels and/or solid fillers to the basecoat compositions.
All these measures, however, have disadvantages.
If, for example, the mean molecular weight of the binder components in established, optimally balanced basecoat compositions containing metallic pigments is reduced, it is true that a part of the organic diluents or solvents which are volatile under processing conditions can be dispensed with, but against that one must put up with a poorer metallic effect, redissolution problems, deterioration of color retention and loss of reliability in application.
The addition of microgels usually affects the rheological properties of the paint systems; in addition, compatibility problems can occur.
Microgels cannot be added to established, optimally balanced basecoat compositions, in particular basecoat compositions containing metallic pigments, without creating further problems. They require an expensive adaptation of the total paint system to the amounts and type of the microgels to be added.
The addition of solid fillers to the basecoat compositions, in particular to basecoat compositions containing metallic pigments, has a negative effect on the appearance of the resultant finishes and usually does not permit any reduction of the absolute amount of components which are volatile under processing conditions, contained in the basecoat compositions.
The present invention is based on the object of providing a process for increasing the proportion of the components which are non-volatile under processing conditions, in basecoat compositions containing predominantly organic diluents or solvents, for the production of multicoat finishes, preferably metallic finishes.
The process should be applicable to as many established, optimally balanced basecoat compositions as possible, in particular to basecoat compositions containing metallic pigments, without any large technical outlay, and it should not affect negatively the quality (for example redissolution behavior, color retention, reliability in application and achievable metallic effect) of the established, optimally balanced basecoat compositions.
Surprisingly, these objects can be achieved by replacing up to 10% by weight of the components which are volatile under processing conditions, by an aliphatic polyether or by mixtures of aliphatic polyethers, the aliphatic polyethers having a mean molecular weight of not less than 300, containing not less than one, preferably ably not less than two, hydroxyl groups per molecule and being liquid under normal conditions.
The invention also relates to the use of aliphatic polyethers which are liquid under normal conditions or of mixtures of aliphatic polyethers, preferably polypropylene oxides, having a mean molecular weight of not less than 300 and containing not less than one, preferably not less than two, hydroxyl groups per molecule, for increasing the proportion of components which are non-volatile under processing conditions, in basecoat compositions containing predominantly organic diluents or solvents, for the production of multicoat finishes, preferably metallic finishes.
The process according to the invention can be employed for all basecoat compositions containing predominantly organic diluents or solvents and containing or not containing microgels, for the production of multicoat finishes, and is suitable for basecoat compositions containing or not containing metallic pigments.
The process according to the invention is preferably used for basecoat compositions containing predominantly organic diluents or solvents, which are suitable for the production of metallic finishes of the basecoat-clearcoat type. Many such basecoat compositions are described in the literature. There are for example the patents U.S. Pat. Nos. 3,639,147, 4,576,868, 4,220,679 and U.S. Pat. No. 4,477,536. Paints containing metallic pigments and based on cellulose acetobutyrate/polyester, cellulose acetobutyrate/acrylate resin, polyurethane/ polyester, microgel/acrylate resin or microgel/polyurethane/polyester binder systems are particularly preferred.
The basecoat compositions under discussion are basecoat compositions which contain non-aqueous organic diluents or solvents. These basecoat compositions are supplied to the paint finisher with a defined on-delivery viscosity which is higher than that required for the application. There the paint is processed in such a way that
(1) it is brought to the viscosity required for the application by the addition of organic solvents or diluents,
(2) it is applied to the substrate by means of generally known application methods (in particular by pneumatic application or by electrostatic high-speed rotary atomization),
(3) it is overcoated with a clearcoat after a flash-off period generally lasting 30 to 500 seconds, and finally
(4) it is baked in general from about 120 to 140xc2x0 C. for 20 minutes.
The term xe2x80x9ccomponents non-volatile under processing conditionsxe2x80x9d is in this context understood to mean paint components which do not lose more than 5% of their weight by volatilization, ie. by evaporation, under conditions prevailing in the processing stages (1) to (4).
The term xe2x80x9caliphatic polyethersxe2x80x9d is understood to mean substances containing several structural units of the general formula xe2x80x94(Rxe2x80x94O)xe2x80x94, in which R represents an alkylene radical of 2 to 6 carbon atoms.
The aliphatic polyethers which can be used according to the invention, must possess a mean molecular weight of not less than 300. Furthermore, they must contain not less than one, preferably not less than two, hydroxyl groups per molecule and they must be liquid under normal conditions (20xc2x0 C., 760 Torr).
Provided that these limitations are observed, any aliphatic polyether which is soluble or dispersible in the organic solvents or diluents can be used for increasing the proportion of components which are non-volatile under processing conditions. Examples of such aliphatic polyethers are polyalkylene oxide polyols, such as for example polyethylene oxide polyols, polypropylene oxide polyols as well as ethoxylation and/or propoxylation products of suitable di- to hexavalent starter molecules, such as for example glycerol, trimethylolpropane, hexanetriol, pentaery-thritol, sorbitol, sucrose, ammonia, ethylenediamine, aniline etc. Mixtures of aliphatic polyethers can also be used. Polypropylene oxide polyols are preferably used.
The addition of the aliphatic polyethers according to the invention is limited by two factors: on the one hand the aliphatic polyethers according to the invention behave in the coatings produced from the basecoat compositions as plasticizers, and on the other hand the aliphatic polyethers according to the invention increase the hydrophilicity of the coatings produced from the basecoat compositions. The extent of the two effects depends as much on the composition of the basecoat compositions as on the nature of the aliphatic polyethers used. The average person skilled in the art can rapidly determine by simple means in what amount a particular aliphatic polyether can be incorporated in a basecoat composition without the coatings produced from it being too soft or too hydrophilic.
Surprisingly, up to 10% by weight of the components which are volatile under processing conditions can be replaced in established, optimally balanced basecoat compositions preferably containing metallic pigments, by the aliphatic polyethers according to the invention without negatively affecting redissolution behavior, color retention, reliability in application or metallic effect of the coatings produced from the basecoat compositions.
The aliphatic polyethers according to the invention probably act as reactive diluents or solvents. They do not have a negative effect on the good properties of optimally balanced basecoat compositions, they are non-volatile under processing conditions and probably react in the baking process via their hydroxyl groups with suitable binder components, for example melamine-formaldehyde resins.
Surprisingly, it has been further found that the addition according to the invention of the aliphatic polyethers according to the invention also improves the spray mist pick-up and facilitates the electrostatic application.
By the process according to the invention a process has been made available which permits to increase, by very simple means, the proportion of components which are non-volatile under processing conditions, in the basecoat compositions under discussion or to adjust it to a given value.